Infection with HCV is a major cause of human liver disease throughout the world. Chronic infection with HCV is associated with chronic liver disease, cirrhosis, hepatocellular carcinoma, and liver failure. HCV is a hepacivirus member of the Flaviviridae family of RNA viruses that affect animals and humans. The genome is a single ˜9.6-kilobase strand of RNA, and consists of one open reading frame that encodes for a polyprotein of ˜3000 amino acids flanked by untranslated regions at both 5′ and 3′ ends (5′- and 3′-UTR). The polyprotein serves as the precursor to at least 10 separate viral proteins critical for replication and assembly of progeny viral particles. The organization of structural and non-structural proteins in the HCV polyprotein is as follows: C-E1-E2-p7-NS2-NS3-NS4a-NS4b-NS5a-NS5b. While the pathology of HCV infection affects mainly the liver, the virus is found in other cell types in the body including peripheral blood lymphocytes.
HCV is major causative agent for post-transfusion and for sporadic hepatitis. Infection by HCV is insidious in a high proportion of chronically infected, and infectious, carriers who may not experience clinical symptoms for many years. An estimated 170 million chronic carriers worldwide are at risk of developing liver disease.
Due to the high degree of variability in the viral surface antigens, existence of multiple viral genotypes, and demonstrated specificity of immunity, the development of a successful vaccine in the near future is unlikely. Alpha-interferon, alone or in combination with ribavirin, has been widely used for treatment of chronic HCV infection. However, treatment of HCV with interferon has frequently been associated with adverse side effects such as fatigue, fever, chills, headache, leukopenia, thrombocytopenia, psychiatric effects and associated disorders, autoimmune phenomena and associated disorder and thyroid dysfunction. Ribavirin, an inhibitor of inosine 5′-monophosphate dehydrogenase (IMPDH), enhances the efficacy of IFN-alpha in the treatment of HCV. Despite the introduction of ribavirin, more than 50% of the patients do not eliminate the virus with the current standard therapy of interferon-alpha (IFN) and ribavirin.
Recently, with the introduction of pegylated interferon, both initial and sustained response rates have improved, and combination treatment of Peg-IFN with ribavirin constitutes the gold standard for therapy. However, the side effects associated with combination therapy persist. Ribavirin causes significant hemolysis in 10-20% of patients treated at currently recommended doses, and the drug is both teratogenic and embryotoxic. Furthermore, a substantial number of patients do not respond with a sustained reduction in viral load, and there is a clear need for more effective antiviral therapy for HCV infection.
A number of approaches are being pursued to combat the virus. These include, for example, application of antisense oligonucleotides or ribozymes for inhibiting HCV replication. Furthermore, low-molecular weight compounds that directly inhibit HCV proteins and interfere with viral replication are considered attractive strategies to control HCV infection. Among the viral targets, the NS3/4a protease/helicase and the NS5b RNA-dependent RNA polymerase are considered the most promising viral targets for new drugs.
Based on the foregoing, there exists a significant need for new effective drugs for treating infections caused by HCV.